1. Field of the Invention
The present invention relates to a focused ion beam (FIB) system of the three-lens type.
2. Description of Related Art
Many focused ion beam systems now available in the market are fundamentally equipped with two lenses: an electrostatic condenser lens for controlling the angular aperture of an ion beam and an electrostatic objective lens for focusing the ion beam onto a specimen.
On the other hand, there exist focused ion beam systems equipped with three lenses, though the number is limited.
FIGS. 4A and 4B show a focused ion beam system of the latter type. The system has an emitter 10 having a front-end portion into which a substance to be ionized is supplied, an extraction electrode 20 for extracting the particles ionized in the front-end portion of the emitter 10 as an ion beam, a condenser lens (first condenser lens) 30 for controlling the angular aperture of the extracted ion beam, a beam aligner 40 for deflecting the ion beam whose angular aperture has been controlled by the condenser lens 30, such as for alignment, a current-limiting aperture (apertured baffle) 50 for extracting an ion beam having a certain angle of radiation from the ion beam whose angular aperture has been controlled by the condenser lens 30, an electrostatic angular aperture control lens (second condenser lens 60 capable of controlling the angular aperture of the ion beam without varying the amount of current of the ion beam passed through the current-limiting aperture 50, a deflector 70 for scanning the ion beam over a specimen 90, and an electrostatic objective lens 80 for focusing the beam, whose aperture angle has been controlled by the control lens 60, onto the specimen 90. That is, this type of focused ion beam system has three lenses: electrostatic condenser lens 30 for controlling the angular aperture of the ion beam, electrostatic angular aperture control lens 60 capable of controlling the angular aperture of the ion beam without varying the amount of current of the ion beam passed through the current-limiting aperture, and electrostatic objective lens 80 for focusing the beam onto the specimen.
In a so-called two-lens type focused ion beam system of the design as described above, with respect to the beam current of the ion beam, only one current is defined in principle per inside diameter of the current-limiting aperture.
Of course, the beam current can be controlled over a considerably wide range by the excitation intensity of the condenser lens. However, there is the problem that the beam diameter deteriorates severely at other than a certain current value. Accordingly, in practical situations, the number of kinds of inside diameter of current-limiting aperture is equal to the number of definable currents.
On the other hand, the focused ion beam system of the three-lens type can control the angular aperture of the ion beam independent of the beam current. Therefore, it is possible to control the beam current over a wide range with one inside diameter of current-limiting aperture without severe deterioration of the beam diameter.
FIG. 5 shows the relation between controllable beam current and beam diameter at each of four different inside diameters #0 to #4 of current-limiting aperture. The broken lines indicate the relations regarding FIB systems of the two-lens type, while the solid lines indicate the relations regarding FIB system of the three-lens type. It can be seen from the diagram that if the beam current is controlled over a wide range in FIB systems of the two-lens type, greater deterioration of beam diameter occurs than in FIB systems of the three-lens type for the same inside diameter of current-limiting aperture. That is, where a FIB system of the three-lens type is used, more kinds of beam current can be specified than where a FIB system of the two-lens type is used. As a result, with a FIB system of the three-lens type, any arbitrary beam current can be specified in applications including micromachining employing sputtering, creation of thin film making use of deposition, maskless gas etching, and SIMS (secondary ion mass spectrometry). The throughput in these works can be expected to be enhanced drastically.
As described so far, the FIB system of the three-lens type is more advantageous than the FIB system of the two-lens type, but the actuality is that there exist only a limited number of FIB systems of the three-lens type.
It is estimated that FIB systems of the three-lens type are rarely used for the following three reasons:
1) It is more inexpensive to increase the number of current-limiting apertures that can be exchanged according to the required beam current types than to increase the number of lenses by one.
2) If the number of lenses is increased, only the number of current types available is increased. It is unlikely that the maximum resolution or maximum current density that is a measure of the performance of the FIB system is improved.
3) The user is obsessed with an idea that it is difficult to adjust the FIB instrument of the three-lens type.
Meanwhile, normal electrostatic lenses include two types: (1) deceleration-type electrostatic lens (hereinafter referred to as the deceleration-type lens) for applying a voltage to a lens electrode such that charged particles are decelerated within a lens and (2) acceleration-type electrostatic lens (hereinafter referred to as the acceleration-type lens) for applying a voltage to a lens electrode such that charged particles are accelerated within a lens.
With a FIB system with an accelerating voltage of 30 kV and equipped with a Ga-LMIS (gallium liquid metal ion source) that is a FIB system currently generally available in the market, it has been computationally confirmed that a beam diameter that is nearly half of the beam diameter where a deceleration-type objective lens is used can be obtained by using an acceleration-type objective lens.
However, most FIB systems actually available in the market use deceleration-type objective lenses. It is considered that the reasons are the following two:
1) When a FIB instrument using a deceleration-type objective lens and a FIB instrument using an acceleration-type objective lens are manufactured and experiments are performed in practice, only slight differences are found between the two types of instruments.
2) Scanning ion microscope (SIM) images obtained by a FIB instrument using a deceleration-type objective lens produce better contrast than images obtained by a FIB instrument using an acceleration-type objective lens. This phenomenon becomes more conspicuous as the current value is reduced by decreasing the diameter of the current-limiting aperture (i.e., as the resolution is increased).
Although a FIB system of the three-lens type in which the angular aperture control lens 60 (second condenser lens) has been replaced by an electrostatic aberration-correcting means has begun to be discussed in papers, this type of instrument has not yet been put on the market.
As described so far, it has been difficult in practice to yield the advantage of the FIB system of the two-lens type (i.e., even if an acceleration-type objective lens is used, aberration coefficient and beam diameter can be made smaller).
We have confirmed that in a case where an acceleration-type objective lens is used, the following problems take place unlike the case where a deceleration-type objective lens is used.
1) As the aperture diameter is reduced, the actually measured value of the beam current deviates more from the calculated value. Furthermore, where a current detector is placed below the current-limiting aperture and the angular current density is measured, as the diameter of the aperture is reduced, the measured angular current density increases disproportionately in spite of the fact that the diameter of the aperture is varied within a range in which the angular current density can be assumed to be constant.
For example, it is assumed that the current-limiting aperture (apertured baffle) is made of molybdenum, the diameter of the aperture is 20 μm, and the thickness of the aperture is 100 μm. Under these conditions, an angular current density that is nearly three times as large as the assumed value is measured. That is, in the minimum beam current region, a diameter that is considerably smaller than the diameter assumed to result in some current value is required in practice.
2) It can be seen that when the dependence of the maximum resolution on the aperture diameter is measured under the same optical conditions except for the aperture diameter, the maximum resolution has a maximum value at some aperture diameter. That is, it follows that the maximum resolution cannot be obtained at the minimum current (minimum aperture diameter). It is known that in a normal FIB system, diffraction aberration can be neglected if ion mass and accelerating voltage are taken into consideration.
The foregoing problems with the FIB system of the three-lens type cannot be explained away by the optics theory. Our earnest research on these problems has led to the discovery that the following phenomena have hindered improvement of the resolution of an acceleration-type objective lens.
1) Beam current Iexp measured with a current detector, such as a PCD, is a superimposition of a main current (Ip (probe current)) based on an ion beam from the emitter and background current (≅Ibck). That is, the following relation holds:Iexp=IP+Ibck 
2) The background current Ibck arises from charged particles produced by sputtering of the ion beam which is emitted from the emitter and which irradiates the edge of the current-limiting aperture. That is, ion beam irradiation of the edge of the aperture produces a large amount of ions having lower energies as compared with the accelerating voltage from the edge.
3) Therefore, where the accelerating voltage is kept constant, the magnitude of the background current Ibck is in proportion to the main current density Jap at the aperture position and to the aperture radius rap of the current-limiting aperture. Furthermore, the magnitude of the background current Ibck depends on the kind of the beam, acceleration, thickness d of the current-limiting aperture, material of the aperture, and incident angle to the aperture. The main current density Jap and the incident angle to the aperture are dependent on the voltage Vcll applied to the condenser lens 30.
Where the kind of the beam and the acceleration are determined, the background current Ibck can be given byIbck=ε·Jap(Vcll)·rap where ε=ε (material, d).
The problems occurring when an acceleration-type objective lens is used as described above can be explained further as follows.
4) Most kinetic energies of charged ions produced by sputtering are considerably lower as compared with the accelerating voltage. As a result, low-energy charged particles of this kind cannot pass through a deceleration-type objective lens acting to decelerate ions. On the other hand, most of low-energy charged ions of this kind can pass through an acceleration type-objective lens acting to accelerate ions. It is considered that this is the cause of the inability to fully bring out the forecasted performance where an acceleration-type objective lens is used.
5) Where an acceleration-type objective lens is used, if the aperture diameter is reduced, the background current will increase rapidly. As a result, subtle contrast will be buried in the background current. The image would be observed as if the resolution deteriorated.
The considerations given so far lead to the conclusion that good results will be obtained by reducing ε so as to reduce the background current Ibck. In this case, ε can be reduced by reducing the thickness d of the current-limiting aperture.
We have fabricated FIB systems using acceleration-type objective lenses. The thickness d of the current-limiting aperture of one system was 100 μm. The thickness d of the other system was 10 μm. Experiments for evaluating the beam current were performed for the same conditions except for the aperture thickness d. A general acceleration-type lens was used as the aperture angle control lens of each of these FIB systems of the three-lens type.
As a result, where the thickness d was 10 μm, the calculated beam current was well in agreement with the measured value even within the minimum current range. The dependence of the angular current density on the aperture diameter disappeared. An assumed given value was derived. It was confirmed that in the case where the thickness d was 10 μm, contrast and resolution of SIM images were much higher than where the thickness d was 100 μm.
However, where the thickness d was 10 μm, it was confirmed that the current-limiting aperture chipped off near its inner wall and the diameter increased after a short time from the beam irradiation. That is, the lifetime of the aperture dropped greatly. The life was about one tenth of the life achieved where the thickness d was a normal value of about 100 μm. This has demonstrated that this instrument has no practicality.
It is considered that the considerations regarding the theoretical advantages and actual problems arising when an acceleration-type objective lens is used in a FIB system of the three-lens type are similarly applied to a FIB system using electrostatic aberration-correcting means instead of an electrostatic angular aperture control lens.